Try to fix an issue where make fails if a header file disappears
Try to fix an issue where make fails if a header file disappears

Currently, when a header file is removed (or renamed), there is a stale
dependency file still referring to the header, and GNU make aborts, because
the header is missing.  Attempt to work around this by adding a pattern
rule to generate any header in `src/`.  The rule prints a warning message.

The C source file whose header was missing will be recompiled because a
dependency was rebuilt.  Once it is, the removed header should be removed
from the dependency file.

(Aside: FWIW, the "ninja" build tool knows directly about depfiles, and it
does not have this problem.)

rprichard/winpty,fracting/winpty,fracting/winpty,rprichard/winpty,fracting/winpty,fracting/winpty,rprichard/winpty,rprichard/winpty